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Researchers assumed the Denisovan ancestry found in Asia was due to migration from Oceania, the larger region containing Melanesia.

What they found was a distinct set of Denisovan ancestry among some modern East Asians—particularly Han Chinese, Chinese Dai and Japanese—ancestry not found in South Asians or Papuans.

Browning and her colleagues assume that modern humans mixed with the Denisovans shortly after migrating out of Africa

While they’re not sure of the location, they believe the interbreeding occurred in at least two places: eastern Asia, and further south, in Indonesia or Australia.

While the new study confirms that modern humans interbred at least three times with ancient hominins—once with Neanderthals, and twice with the Denisovans

Comparing genomes, scientists concluded that today’s humans outside Africa carry an average of 2.5 percent Neanderthal DNA, and that people from parts of Oceania also carry about 5 percent Denisovan DNA. A study published in November found that Southeast Asians carry about 1 percent Denisovan DNA in addition to their Neanderthal genes.

as few as six couplings all those tens of thousands of years ago might have led to the current level of ancient immune alleles.

This means that our modern era, since H. floresiensis died out, is the only time in the four-million-year human history that just one type of human has been alive,

Were they romantic couplings? More likely they were aggressive acts between competing human groups, Dr. Stringer said. For a model, he pointed to modern hunter-gatherer groups that display aggressive behavior among tribes.

The value of the interbreeding shows up in the immune system, Dr. Parham’s analysis suggests. The Neanderthals and Denisovans had lived in Europe and Asia for many thousands of years before modern humans showed up and had developed ways to fight the diseases there

When modern humans mated with them, they got an injection of helpful genetic immune material, so useful that it remains in the genome today. This suggests that modern humans needed the archaic DNA to survive. The downside of archaic immune material is that it may be responsible for autoimmune diseases like diabetes, arthritis and multiple sclerosis, Dr. Parham said, stressing that these are preliminary results.

little is known about the Denisovans — the only remains so far are the pinky bone and the tooth, and there are no artifacts like tools. Dr. Reich and others suggest that they were once scattered widely across Asia, from the cold northern cave to the tropical south. The evidence is that modern populations in Oceania, including aboriginal Australians, carry Denisovan genes.

They coexisted in Europe for thousands of years, apparently trading tools and sharing cultures. Some groups survived the Ice Age, while others vanished,

when farmers arrived in Europe about 8,000 years ago, they encountered the descendants of this long history, with light-skinned, dark-eyed people to the east, and possibly dark-skinned and blue-eyed people to the west.

“We lack still an understanding of why these movements were triggered. What happened here, why it happened — it’s strange.”

Modern humans arose in Africa and expanded to other continents about 60,000 years ago

These early Europeans have almost no genetic link to younger remains of hunter-gatherers. It appears that the first modern humans in Europe may have disappeared along with the Neanderthals

he oldest DNA of modern humans in Europe, dating back 45,000 years, undermines such a simple story. It comes from people who belonged to a lost branch of the human family tree. Their ancestors were part of the expansion out of Africa, but they split off on their own before the ancestors of living Europeans and Asians split apart.

About 33,000 years ago, as the climate turned cold, a new culture called the Gravettian arose across Europe. Gravettian hunters made spears to kill woolly mammoths and other big game. They also made so-called Venus figurines that might have represented fertility.

When the glaciers retreated, some descendants of the Fournol continued living in Iberia. But others expanded north as a new population, which Dr. Posth and his colleagues called GoyetQ2. “It really seems like a peopling of Europe after the last glacial maximum,

Dr. Posth and his colleagues found DNA in Gravettian remains scattered across Europe. The scientists had expected all of the individuals to have come from the same genetic population, but instead found two distinct groups: one in France and Spain, and another in Italy, the Czech Republic and Germany.

“They were very distinct, and this was a very big surprise to us because they practiced the same archaeological culture,”

Dr. Posth and his colleagues named the western population the Fournol people, and found a genetic link between this group and 35,000-year-old Aurignacian remains in Belgium.

They called the eastern group Vestonice, and discovered that they share an ancestry with 34,000-year-old hunter-gatherers who lived in Russia.

That genetic gulf led Dr. Posth and his colleagues to argue that the Fournol and Vestonice belonged to two waves that migrated into Europe separately. After they arrived, they lived for several thousand years sharing the Gravettian culture but remaining genetically distinct.

It’s clear from the new study that they were not isolated entirely from each other. In Belgium, the scientists found 30,000-year-old remains with a mix of Fournol and Vestonice ancestry.

About 26,000 years ago, the two groups faced a new threat to their survival: an advancing wall of glaciers. During the Ice Age, from 26,000 to 19,000 years ago, European hunter-gatherers were shut out of much of the continent, surviving only in southern refuges.

the refuge of the Iberian Peninsula, the region now occupied by Spain and Portugal, by studying DNA in the teeth of a 23,000-year-old man found in a cave in southern Spain. His DNA revealed that he belonged to the Fournol people who lived in Iberia before the Ice Age. The researchers also found genetic markers linking him to a 45,000-year-old skeleton discovered in Bulgaria.

When these groups arrived in Europe, Neanderthals had already been living across the continent for more than 100,000 years. The Neanderthals disappeared about 40,000 years ago, perhaps because modern humans outcompeted them with superior tools.

The Vestonice, by contrast, did not survive the Ice Age. When the glaciers were at their most expansive, the Vestonice may have endured for a time in Italy. But Dr. Posth and his colleagues found no Vestonice ancestry in Europeans after the Ice Age. Instead, they discovered a population of hunter-gatherers that appeared to have expanded from the Balkans, known as the Villabruna. They moved into Italy and replaced the Vestonice.

For several thousand years, the Villabruna were limited to southern Europe. Then, 14,000 years ago, they crossed the Alps and encountered the GoyetQ2 people to the north. A new population emerged, its ancestry three parts Villabruna to one part GoyetQ2.

This new people, which Dr. Posth and his colleagues called Oberkassel, expanded across much of Europe, replacing the old GoyetQ2 population.

another climate shift could explain this new wave. About 14,000 years ago, a pulse of strong warming produced forests across much of Europe. The Oberkassel people may have been better at hunting in forests, whereas the GoyetQ2 retreated with the shrinking steppes.

To the east, the Oberkassel ran into a new group of hunter-gatherers, who probably arrived from Russia. The scientists named this group’s descendants, who lived in Ukraine and surrounding regions, the Sidelkino.

in Iberia, there were no great sweeps of newcomers replacing older peoples. The Iberians after the Ice Age still carried a great deal of ancestry from the Fournol people who had arrived there thousands of years before the glaciers advanced. The Villabruna people moved into northern Spain, but added their DNA to the mix rather than replacing those who were there before.

When the first farmers arrived in Europe from Turkey about 8,000 years ago, three large groups of hunter-gatherers thrived across Europe: the Iberians, the Oberkassel and the Sidelkino. Living Europeans carry some of their gene

The Sidelkino people in the east had genes associated with dark eyes and light skin. The Oberkassel in the west, in contrast, probably had blue eyes and may have had dark skin

These three groups of hunter-gatherers remained isolated from each other for about 6,000 years, until the farmers from Turkey arrived. After this advent of agriculture, the three groups began mixing, the scientists found. It’s possible that the spread of farmland forced them to move to the margins of Europe to survive. But over time, they were absorbed into the agricultural communities that surrounded them.

every continent will likely have its own history of hunter-gatherer migrations.

it is now possible to extract human DNA from cave sediments rather than searching for bones and teeth.